Rename opsim directory to obstable

[jeremykubica]

Make the naming of the obstable directory consistent with the new base class (ObsTable).

Also rename ZTFOpSim to ZTFObsTable.

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Codecov Report

❌ Patch coverage is 80.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 91.50%. Comparing base (4e7e3be) to head (e8983d0).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/tdastro/obstable/ztf_obstable.py	75.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #429   +/-   ##
=======================================
  Coverage   91.50%   91.50%           
=======================================
  Files          53       53           
  Lines        3767     3767           
=======================================
  Hits         3447     3447           
  Misses        320      320           

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Before [4e7e3be]	After [8933bc7]	Ratio	Benchmark (Parameter)
738±40μs	668±30μs	~0.91	benchmarks.TimeSuite.time_fnu_to_flam
9.43±0.5ms	9.73±0.6ms	1.03	benchmarks.TimeSuite.time_evaluate_salt3_model
33.2±0.2ms	33.6±0.4ms	1.01	benchmarks.TimeSuite.time_additive_multi_model_source
4.69±0.03ms	4.75±0.03ms	1.01	benchmarks.TimeSuite.time_lightcurve_source
8.37±0.06ms	8.41±0.06ms	1.01	benchmarks.TimeSuite.time_load_passbands
88.2±2μs	88.9±1μs	1.01	benchmarks.TimeSuite.time_make_and_evaluate_step_model
11.0±0.07ms	11.2±0.2ms	1.01	benchmarks.TimeSuite.time_make_evaluate_constant_sed_model
12.4±0.2μs	12.5±0.2μs	1.01	benchmarks.TimeSuite.time_sample_x1_from_hostmass
1.36±0.05ms	1.37±0.03ms	1.00	benchmarks.TimeSuite.time_apply_passbands
7.66±0.02ms	7.65±0.2ms	1.00	benchmarks.TimeSuite.time_apply_white_noise

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